Where is an error?
Skydiver.sm (14 KiB) downloaded 62 time(s).

Must be some plot

Hi Valery. You can't just 'sterilize' the units because coolprops needs them. This could be an approach.

Skydiver.sm (21 KiB) downloaded 56 time(s).



Best regards.
Alvaro.

Thanks!
But the problem in if
Try please
Skydiver-pv-RT.sm (18 KiB) downloaded 48 time(s).
The parachutist is now falling towards the ground. His parachute won't open. We have to save him!

We saved the skydiver!
Skydiver-OK.sm (23 KiB) downloaded 52 time(s).


https://yandex.ru/video/preview/420757228716091438

This also works:

Skydiver-OK_b.sm (22 KiB) downloaded 54 time(s).

Wrote

This also works:

Skydiver-OK_b.sm (22 KiB) downloaded 54 time(s).

... and this too
Skydiver-OK_c.sm (20 KiB) downloaded 50 time(s).

Why with if we have an error?
Why with cases no an error?
Try please the attach.
Is it some bug?
Skydiver-If-Cases.sm (26 KiB) downloaded 60 time(s).

Wrote

Why with if we have an error?
Why with cases no an error?
Try please the attach.
Is it some bug?

It is probably related to some preprocessing, the function if() is a numerical function and returns an error if the arguments are not defined.

It's the same in integrals, you can't use functions like if() because at some level a preprocessing is required and the function is executed with unknowns as arguments (to retrieve the function from the global context, or some other feature like guessing the unknowns or the units).

cases() works in a different way and you can use for these tasks, but it come at a the cost of not being able to use definitions inside its arguments, because each single case it's not "sealed" from preprocessing by the error that if() throws.